In the blow molding of polyethylene a processing problem can occur in which the parison will stick to the die momentarily prior to blowing an article such as a bottle. This condition is referred to as “cuffing”, “curling”, or “parison sticking”. Minor parison cuffing can lead to problems such as marking or scoring of portions of the article, such as the base of the bottle, or forming of a bead of material on the inside seam of the article. In the most severe cases, cuffing can cause failure to properly form the article.
Parison cuffing is usually attributed to blow molding machine conditions such as damaged or mismatched tooling or improperly centered pressure rings. One way to solve the problem of cuffing is to perform special maintenance or conduct repair work to the head in which the cuffing occurs. This technique does not always eliminate the occurrence of cuffing.
It is known from CA 961998 to protect against gel streak formation in anti-oxidant stabilized polyolefin films by adding to the extrusion composition 0.01 to 0.1 wt. % polyalkylene glycol, based on the weight of the polyolefin.
U.S. Pat. No. 4,013,622 teach the reduction of breakdown in a film formed from low density polyethylene resin with the addition to the resin of 200-500 ppm polyethylene glycol having a molecular weight of 1,300-7,500.
More generally, U.S. Pat. No. 6,294,604 teaches desirable surface characteristics on extruded polymers by the use of fluorocarbon polymers as an additive with poly(oxyalkylene)polymers.
Carbowax™ PEG 300 is taught as a possible additive to a film comprising HDPE to “inhibit initial coloration” in U.S. patent application Ser. No. 2003/0225194.
The present inventors have surprisingly found that the addition of a small amount of a low molecular weight polyethylene glycol likewise reduces or eliminates parison cuffing in a blow molding process. The present inventors have also surprisingly found that lowering the melt index of the polymer used in the process reduces or eliminates the occurrence of parison cuffing. By combining these two features, a more robust blow molding composition is formed to provide an improved blow molding process whereby parison cuffing defects are reduced or eliminated.